1. Field of the Invention
The present invention relates to a method for manufacturing a surface mount device (SMD)-type piezoelectric vibrator in which a piezoelectric vibrating reed is sealed in a cavity that is formed between two bonded substrates, a piezoelectric vibrator manufactured by the manufacturing method, and an oscillator, an electronic device, and a radio-controlled timepiece each having the piezoelectric vibrator.
2. Description of the Related Art
Recently, a piezoelectric vibrator utilizing quartz or the like has been used in cellular phones and portable information terminals as a time source, a timing source of a control signal, a reference signal source, and the like. There are various piezoelectric vibrators of this type, and a surface mount device-type piezoelectric vibrator is one known example thereof. As a piezoelectric vibrator of this type, a three-layered piezoelectric vibrator in which a piezoelectric substrate having a piezoelectric vibrating reed formed thereon is bonded to a base substrate and a lid substrate so as to be interposed therebetween is generally known. In this case, the piezoelectric vibrator is accommodated in a cavity (sealed space) that is formed between the base substrate and the lid substrate. Moreover, in recent years, instead of the three-layered piezoelectric vibrator described above, a two-layered piezoelectric vibrator has also been developed.
The piezoelectric vibrator of this type has a two-layered structure in which a base substrate and a lid substrate are directly bonded, and a piezoelectric vibrating reed is accommodated in a cavity formed between the two substrates.
The two-layered piezoelectric vibrator is ideally used as it is superior in achieving a thin profile as compared with the three-layered structure. As an example of such a two-layered piezoelectric vibrator, there is known a piezoelectric vibrator in which using a conductive member formed so as to penetrate through the base substrate, the piezoelectric vibrating reed is electrically connected to the outer electrodes formed on the base substrate (see Patent Citations 1 and 2).
As shown in FIGS. 33 and 34, this piezoelectric vibrator 200 includes a base substrate 201 and a lid substrate 202 which are anodically bonded to each other with a bonding film 207 disposed therebetween, and a piezoelectric vibrating reed 203 which is sealed in a cavity C formed between the two substrates 201 and 202. The piezoelectric vibrating reed 203 is a tuning-fork vibrating reed, for example, and is mounted on the upper surface of the base substrate 201 inside the cavity C by a conductive adhesive agent E.
The base substrate 201 and the lid substrate 202 are insulating substrates formed, for example, from ceramics, glass, and the like. On the base substrate 201 among the two substrates 201 and 202, through-holes 204 are formed so as to penetrate through the substrate 201. Inside the through-holes 204, a conductive member 205 is buried so as to close the through-holes 204. The conductive member 205 is electrically connected to outer electrodes 206 which are formed on the lower surface of the base substrate 201 and are also electrically connected to the piezoelectric vibrating reed 203 which is mounted inside the cavity C.
Patent Citation 1: JP-A-2002-124845
Patent Citation 2: JP-A-2006-279872
However, in the two-layered piezoelectric vibrator described above, the conductive member 205 has two major roles: one is to close the through-holes 204 to maintain the air-tightness of the inside of the cavity C, and the other is to electrically connect the piezoelectric vibrating reed 203 and the outer electrodes 206 to each other. Particularly, if the adhesion to the through-holes 204 is not sufficient, there is a problem in that the air-tightness of the inside of the cavity C is degraded. Moreover, if the contact with the conductive adhesive agent E or the outer electrodes 206 is not sufficient, there is a problem in that operation errors of the piezoelectric vibrating reed 203 may be caused. Therefore, in order to obviate such a problem, it is necessary to form the conductive member 205 so as to completely close the through-holes 204 in a state of being tightly adhered to the inner surfaces of the through-holes 204 and in a state where no depression or the like is present on the surface thereof.
Although Patent Citations 1 and 2 disclose the use of the conductive member 205 which is formed from a conductive paste (an Ag paste, an Au—Sn paste, or the like), there is no disclosure as to a specific manufacturing method of how the conductive member 205 was actually formed.
In general, when a conductive paste is used, it needs to be baked and cured. That is, it is necessary to bury a conductive paste in the through-holes 204 and then cure the conductive paste by baking. However, if baking is performed, an organic material included in the conductive paste will disappear by evaporation. Therefore, the volume of the conductive paste after the baking will generally decrease as compared to before the baking (for example, when an Ag paste was used as the conductive paste, the volume decreased by approximately 20%). For this reason, even when the conductive member 205 was formed using the conductive paste, there is a problem in that depressions are formed on the surface thereof, and in severe cases, a penetration hole is opened at the center thereof.
As a result, there is a possibility that the air-tightness of the inside of the cavity C is degraded and the conduction between the piezoelectric vibrating reed 203 and the outer electrodes 206 is degraded.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a piezoelectric vibrator manufacturing method for manufacturing a high-quality two-layered surface mount device-type piezoelectric vibrator in which reliable air-tightness of the inside of the cavity is maintained, and stable electrical connection between the piezoelectric vibrating reed and the outer electrodes is secured. Another object of the present invention is to provide a piezoelectric vibrator manufactured by the manufacturing method and an oscillator, an electronic device, and a radio-controlled timepiece each including the piezoelectric vibrator.